1. Field of the Invention
The present invention relates to energy conversion systems for deriving useful power from sources of low level heat such as thermal gradients in the ocean, or from solar, geothermal or other sources of low level heat and more particularly by using warm water to heat a gas, expanding the gas to extract power and then cooling and compressing the gas with cooler water back to initial conditions.
2. Description of the Prior Art
Designs now being proposed for Ocean Thermal Energy Conversions (OTEC) plants nearly all use conventional tubular heat exchangers to vaporize a working fluid, such as ammonia, using heat from a warm water source, then employing the ammonia vapor to drive a turbine, and thereafter condensing the ammonia vapors by indirect exchange with cold water brought up from the bottom. An early "open cycle" design used water vapor from the warm water as the working fluid in order to avoid the very expensive and troublesome indirect heat exchangers, but this system requires an extremely large turbine. Designs using ammonia or other working fluid are now receiving intensive attention and study, and now appear to be generally preferred.
However, these designs all require enormous surface areas in tubular or other indirect heat exchangers because the operation inherently has a very low conversion efficiency, typically 2 to 3%, and so enormous amounts of heat must be transferred from water to ammonia or other working fluid. Moreover, this heat must be transferred at very small temperature differences, such as 2.degree. to 5.degree. F. to conserve the meager difference available between warm water at perhaps 80.degree. and cold water at about 40.degree.. For comparison, a conventional power plant will have 2000.degree. or more on the heat source and roughly 100.degree. on the cold receiver. For an OTEC plant many times more surface area will be required, presenting major problems with regard to cost, fouling/cleaning, and corrosion in an ocean environment, as well as the possibility of serious leaks. Titanium tubing has been proposed, using a wall thickness of only 0.03 inches to avoid intolerable cost, but this poses a major risk as regards mechanical integrity of equipment in such a hostile environment as the ocean.
While the present OTEC designs promise to provide power from an inexhaustible source at comparative cost and with minimum harm to the environment, there is considerable room for improvement in the areas of cost, reliability, fouling and maintenance, and materials of construction problems. Plastics are of great interest in that they are low-cost, easy to fabricate, and not attacked by salt water. However, they have not been suitable for heat exhcnagers because of poor heat transfer properties. Heat exchangers are the largest, most costly, and most critical part of conventional systems.
Theoretically, the maximum efficiency possible for such energy conversion is given by the relationship: EQU E=(T.sub.1 -T.sub.2)/T.sub.1
where E is fractional efficiency, T.sub.1 is temperature of the warm source, and T.sub.2 is temperature of the cold receiver. For a typical OTEC case having 80.degree. warm water and 40.degree. cold water, the theoretical maximum efficiency is 7.4%. Actual designs can achieve only 2.5 to 3% efficiency, allowing for practical heat exchangers and "parasitic" power losses for water pumps and other auxiliaries.
In general, the designs being pursued are based on evaporating a liquid at about constant pressure, with the result that all of the heat input takes place at nearly constant temperature, thus preventing the application of countercurrent or crossflow heat exchange over a maximum practical temperature range. Moreover, the latent heat of condensation is finally rejected to the cold receiver, again at nearly constant temperature.